1. Field
The present disclosure relates to a pressure sensor, and particularly relates to a pressure sensor including a detecting device that includes a diaphragm that is displaced in response to pressure from a fluid.
2. Description of the Related Art
A pressure sensor, such as a capacitance diaphragm gauge, includes a detecting device that includes a diaphragm. With the detecting device attached to a pipe or the like through which a gas to be measured flows, the pressure sensor converts the amount of deflection (or displacement) of the diaphragm under pressure into a capacitance value and outputs a pressure value determined from the capacitance value. This pressure sensor is less dependent on the type of gas and thus is used, for example, in semiconductor facilities and a wide range of industrial applications (see, e.g., Japanese Unexamined Patent Application Publication No. 2014-109484).
As illustrated in FIG. 4, a detecting device of a pressure sensor, such as a diaphragm gauge, includes a diaphragm 302 configured to receive pressure from an object to be measured, and a base 301 provided with a recess in the center thereof in plan view and having a support portion 301a that supports the diaphragm 302. The diaphragm 302 and the base 301 define a capacitance chamber 303. In the diaphragm 302 supported by the support portion 301a, a movable region 302a spaced from the base 301 can be displaced in the direction of the base 301. The diaphragm 302 and the base 301 are made of an insulating material, such as sapphire.
The detecting device of the pressure sensor includes a movable electrode 304 formed in the movable region 302a of the diaphragm 302, and a fixed electrode 305 formed on the base 301 and facing the movable electrode 304. The detecting device of the pressure sensor further includes a movable reference electrode 306 formed around the movable electrode 304 in the movable region 302a of the diaphragm 302, and a fixed reference electrode 307 formed on the base 301 around the fixed electrode 305 and facing the movable reference electrode 306.
The detecting device of the pressure sensor described above needs not only to be resistant to corrosion by a gas used in an apparatus to which the pressure sensor is attached, but also to be resistant to byproducts produced during the process of film deposition or the like. Byproducts produced during the film deposition process are deposited on areas exposed to a raw material gas. Examples of the areas include the inner wall of a film deposition chamber, the inner wall of a pipe, the interior of a vacuum pump, and the diaphragm serving as a pressure receiver of the pressure sensor. For example, as illustrated in FIG. 4, a byproduct 321 is deposited on the diaphragm 302.
For example, in atomic layer deposition (ALD) used for forming gate insulators, byproducts are deposited, because of their properties, on various areas exposed to a raw material gas. To prevent deposition of such byproducts, for example, each part of a film depositing apparatus where byproducts tend to deposit is heated to about 200° C. during a film deposition operation.
For example, the pressure sensor heats the detecting device to reduce deposition of byproducts, whereas the film depositing apparatus performs similar heating using a heater which is attached to a pipe portion for introducing pressure to the diaphragm of the pressure sensor.
The pressure sensor has a sensitivity to temperature changes (temperature characteristic) (see, e.g., Ichida, Shunji et al. (1991). Development of SPS300 Intelligent Pressure Sensor. Savemation Review, vol. 9, no. 1, pp. 8-14). Therefore, assembly of the pressure sensor is normally followed by evaluation of the temperature characteristic. Then, a measurement circuit, which is configured to correct the output of the pressure sensor in accordance with the temperature at which the detecting device is heated, is adjusted in such a manner that the impact of temperature changes is reduced. This adjustment is followed by shipment of the pressure sensor.
However, even when the heating described above is performed as a preventive measure, byproducts are deposited little by little. The deposition results in changes in the mechanical and thermal characteristics of the diaphragm. Since the byproducts differ from materials forming the diaphragm, the sensitivity of the detecting device to temperature is changed.
The temperature characteristic of the detecting device is changed not only by formation of byproducts, but also by alteration of the surface of the diaphragm caused by exposure to a corrosive gas for etching. Specifically, the temperature characteristic is changed when the diaphragm is etched similarly to a substrate to be processed. The temperature characteristic is also changed when the surface of the diaphragm chemically reacts and changes its properties. The mechanical characteristics of the diaphragm are also changed, for example, by an impact transmitted through a joint or welded portion for securing the diaphragm and a package of the detecting device.
When the mechanical balance of the diaphragm serving as a pressure receiver is changed by stress relaxation or thermal deformation in such a manner as described above, the temperature characteristic is changed and made different from that when adjusted. In this case, the result of measurement made by the pressure sensor differs from actual one and significantly affects the process quality.
As described above, when the temperature characteristic of the detecting device is changed and the pressure sensor is unable to properly perform measurement, the detecting device is recognized as faulty. The faulty detecting device is typically removed from the apparatus and replaced. For replacement of the detecting device, the apparatus needs to be shut down for many hours and this leads to a significant decrease in productivity.
Even when the detecting device recognized as faulty, if it is due to change in temperature characteristic and the adjustment of the temperature characteristic described above is carried out again, the detecting device can be used continuously without being replaced. However, in the related art, adjustment of the temperature characteristic involves removing the detecting device from the apparatus, and this leads to a significant decrease in productivity as described above.